142 



Alma Howaed and S. R. Pelc 



mitotic rate is very high in the final 2 mm. of the tip and 

 decreases with distance from it. In order to discover the 

 percentage of positive cells in relation to their distance from 

 the meristem, longitudinal sections were made of roots grown 

 for 24 and 48 hours in 0-2 /xC. per cc. of ^^P. The resolving 

 power in sections is not as good as that in squashes due to 

 greater thickness, overlapping of cells, and absence of the 

 spreading which is achieved in squashes. However, it is 



40n 



3-0- 



RATIO 



20- 



l-O- 



I 1 



I I I 



O 20 40 60 80 lOO I20-200 



LENGTH OF CELLS p 

 20 HRS. P^^ 



-t— 

 O 



20 40 60 80 lOO I20-200 



LENGTH OF CELLS >j 

 40 HRS P^^ 



Fig. 5. Percentage of cells showing autoradiograph in relation 



to cell length. The indicated standard deviations are based 



on the number of observations only. 



possible adequately to resolve individual nuclei in the elon- 

 gating cells of these roots. The results are summarized in 

 Fig. 6, which gives readings for two roots, one of which had 

 24 hours in ^^p and during this time grew 3-0 cm., while the 

 other had 48 hours treatment and grew 4-2 cm. It will be 

 seen that in the cortical cells the percentage of positive 

 nuclei falls towards zero in both roots, from a level similar 

 to that found in the meristem in squashes. In the developing 

 vascular region, where mitotic activity is appreciable at more 

 proximal levels, the meristematic value is maintained for 



